Absorption activity and barrier properties in the endolymphatic sac. Ultrastructural and morphometric analysis.

A constant volume of horseradish peroxidase (HRP) was injected directly into the endolymphatic sac (ES) lumen of the guinea pig to investigate the detailed absorption activity and the barrier properties of the ES. The reaction products were analyzed using an ultrastructural and morphometric method 1 to 10 h after the injection of this tracer. The epithelial cells in the proximal portion did not absorb the intraluminal HRP at any intervals after the tracer injection. The epithelial cells of the intermediate portion were classified clearly into two types according to their absorption activity: active-absorptive, and non-active cells. Uptake in the active-absorptive cells reached its maximal rate 8 h after the injection and then decreased. The active-absorptive cells in teh intermediate portion are considered to play a major role in the macromolecular absorption in the ES epithelium. The non-active cells scarcely absorbed the intraluminal HRP, suggesting that these cells are not involved in the macromolecular absorption. The absorption activity in the distal portion was lower than that in the active-absorptive cells in the intermediate portion 1 to 8 h after the HRP injection, while higher 10 h after the injection. Not only the intermediate portion but also the distal portion may play an active role in the macromolecular absorption. In no portion of the ES did intraluminal HRP penetrate beyond the junctional complexes between epithelial cells or through the cytoplasm. It is conceivable that there is a tight barrier to the intraluminal macromolecules in the epithelial linings of the ES.